Thinner snow, colder roots – boreal trees show unique cold responses

In this interview, Tuuli Aro speaks about her PhD studies and what her results could mean for forest resilience and tree breeding in a changing climate.

- You did your PhD in Vaughan Hurry’s research group at UPSC in collaboration with Stora Enso and Skogforsk and studied low temperature tolerance in boreal trees. What aroused your interest in this topic?

What intrigued me most was the applied aspect of the research topic and its direct relevance for forestry. The PhD project combined several subprojects that approached cold stress in boreal trees from different perspectives. Nordic forests have a special place in my heart, and their ecological, economic and societal importance and multidimensional role in the Nordic societies makes their research particularly fascinating. As an industrial PhD student, I also had the opportunity to work closely with people from forestry companies, which gave me valuable insights into the industrial and applied perspective of forestry research.

- How do low temperatures challenge the resilience of boreal forests?

Somewhat counterintuitively, a warming climate is making low temperature events even more hazardous for developing plant tissues. Boreal plants are well adapted to cold through acclimation, but changing climate is increasing the risk of low temperature stress outside their acclimation stages, especially in spring, when they have started already to grow. We studied cold responses in fine roots, which are normally insulated by snow during winter. However, reduced snow cover is expected to increase soil freezing, which can damage roots and affect tree growth and carbon cycle.

- When summarising your thesis, what do you consider as the major outcome? 

We aimed to understand how genes in boreal tree roots respond to cold stress, focusing on aspen and birch as broadleaf species and spruce and pine as conifers. One major outcome was that cold-responsive genes show surprisingly low conservation across species. This means that results from one model species cannot easily be transferred to another, specifically if species are distantly related. This highlights the need for improved genetic and molecular tools for both conifer and broadleaf boreal trees, to better understand how they will respond to future climate conditions and to support the development of more climate-resilient trees through genomic methods. 

- Were there any results that you did not expect or that were astonishing you?

Yes, absolutely. In our study on spring frost tolerance prediction in Norway spruce, we found that trees which start growing earlier in spring actually have higher frost tolerance than those that begin growth later. At first, this was unexpected, since in tree breeding, genotypes that delay growth are typically selected to avoid frost damage. But from an evolutionary perspective it makes sense that trees which start growing earlier have developed stronger frost hardiness mechanism.  

- Doing a PhD is often very challenging. Would you like to share some of the challenges that you had to overcome? 

There were many, but the greatest challenge was managing the uncertainty and balancing several subprojects while making sure they formed a coherent thesis. Learning to prioritize effectively was essential. Working remotely for much of my PhD also tested my ability to self-manage and stay motivated, but it taught me valuable skills in independence.

- You worked in close collaboration with Stora Enso and Skogforsk, the Forestry Research Institute of Sweden. What impact has your research on the forest industry? 

Across all of the projects, we demonstrated the importance of fundamental science to applied forestry and how this relationship is needed to understand how trees will grow in a changing climate. Our findings suggest that fine roots may struggle to adapt to colder soil temperatures and this might require adjustments e.g. for seedling establishment to make sure the seedlings survive. We also applied a standard cold stress testing method that had not previously been used in forest breeding and showed that incorporating abiotic stress tolerance traits, such as spring frost tolerance, into genomic selection could enhance future tree breeding. Finally, our discovery of links between frost tolerance and bud burst, highlights how fundamental science can help identify overlooked traits that are relevant in the context of climate change and complement applied research.

- What are your plans now after finishing your PhD? 

I definitely would like to continue to work with forests, and considering the fast temperature increase  specifically in Northern Europe, I would like to contribute to the work on the effects of future climate on Nordic forests.